Allow to delete breakpoint #25
Description
If "set breakpoint" is clicked on a line with breakpoint, toggle it.
Note that it currently possible to set multiple breakpoints on one line in different columns.
Should we toggle breakpoint only is cursor is "near enough" to existing bp? ( this might be tricky, because "near enough" to me is "if set here, actual_position would be exactly the same" so it requires extra test "setBreakpoint").
Pseudo algorithm:
get breakpoints
set at current cursor position
if actual_position overlaps with existing breakpoint, delete both + delete overlapping breakpoint marker.
otherwise, add breakpoint marker